Decoking of onstream thermal cracking tubes with h20 and h2

ABSTRACT

THERMAL CRACKING OF HYDROCARBONS IN ADMIXTURE WITH STEAM IN TUBES ARRANGED IN A CRACKING FURNACE LEADS TO THE DEPOSITION OF COKE ON THE INTERIOR WALLS OF THE TUBES, WHICH COKE MUST BE PERIODICALLY REMOVED IN ORDER TO MAINTAIN CRACKING EFFECIENCY; THE COKE CAN BE REMOVED, WITHOUT SHUTTING DOWN THE FURNACE, BY CUTTING OUT THE FEED TO AT LEAST ONE TUBE AND PASSING THROUGH SUCH TUBE OR TUBES A DECOKING FEED OF STREAM AND/OR WATER AND HYDROGEN, WHILE MAINTAINING THE FURNACE ONSTREAM AND CONTINUING THE THERMAL CRACKING PROCESS IN TUBES THAT ARE NOT BEING DECOKED.

2 sheets-sheet 1 J. A. KIVLEN ETAL Jan. 19, 1971 DECOKING oF oNsT'REAMTHERMAL 'CRACKING TUBES WITH H20 AND H3 Filed oci. 1e, 1968 Jan. 19,1971 .I, A. KIvLEN ETAI. 3,557,241

DECOKING OF ONSTREAM THERMAL CRACKING TUBES WITH H30 AND H2 Filed Oct.16, 1968 2 Sheets-Sheet 2 FIGURE 2 DEcoKING RATE IN H2- H20 GAS MIxTuRESAT ATMOSPHERIC PRESSURE cc co/'SEQ H2O MOLE FRACTION j. K lnvenors BYAttorney United States Patent O 3,557,241 DECOKING F ONSTREAM THERMALCRACKING TUBES WITH H AND H2 John A. Kivlen, Sparta, and Ihor Koszman,Parsippany,

NJ., assignors to Esso Research and Engineering Company, a corporationof Delaware Filed Oct. 16, 1968, Ser. No. 768,065 Int. Cl. C07c 3/00;C10g 9/12 U.S. Cl. 260-683 11 Claims ABSTRACT OF THE DISCLOSURE Thermalcracking of hydrocarbons in admixture with steam in tubes arranged in acracking furnace leads to the deposition of coke on the interior wallsof the tubes, which coke must be periodically removed in order tomaintain cracking efciency; the coke can be removed, without shuttingdown the furnace, by cutting out the feed to at least one tube andpassing through such tube or tubes a decoking feed of steam and/ orwater and hydrogen, while maintaining the furnace onstream andcontinuing the thermal cracking process in tubes that are not beingdecoked.

FIELD OF THE INVENTION This invention relates to a process for decokingthe tubes of a cracking furnace. More particularly, this inventionrelates to an improved process for decoking steam cracking tubes whilemaintaining the furnace onstream and continuing the cracking process.Still more particularly, this invention relates to an onstream decokingprocess whereby the hydrocarbon feed to at least one tube in thecracking furnace is cut out and a decoking feed comprising steam and/orwater and hydrogen is passed through said tube or tubes and the coke isremoved.

PRIOR ART The thermal cracking of petroleum feed streams, with orwithout the presence of steam, is well known to the art and is widelyused as a source of valuable unsaturated compounds, e.g., ethylene,propylene, butadiene. Generally, when noncatalytic processes areconducted, it is desirable to employ steam as the principal diluent inorder to control the reaction and reduce erosion and corrosion effects.While steam cracking has been technically and economically successful,several major drawbacks exist which militate against the development ofthe full potential of the steam cracking process. Essentially, thesedrawbacks center around the carbon forming tendency of the process gas,i.e., vaporized petroleum feed at reaction (cracking) temperatures.

Perhaps the most objectionable drawback relating to carbon formation isthe deposition of coke on the interior of the tube walls through whichthe cracking mixture ows. Coke deposition is believed to be due to theformation of free radicals, e.g., when ethane is cracked methyleneradicals can -be formed, which may then polymerize with other unsaturatecomponents into long chain compounds and dehydrogenate to form coke onthe tube walls. The coke tends to build up and, therefore, reduces theeffective cross-sectional area of the tube necessitating higherpressures to maintain a constant throughput. More importantly, however,because coke is an excellent thermal insulator, its formation isaccompanied by a sharp increase in furnace tube temperature, in order tomaintain l 3,557,241 ce Patented Jan. 19, 1971 cracking efficiency,thereby resulting in a decrease in tube life and the limiting of thecraking temperature that can be employed (which also limits conversionsand yields). Eventually, the coke lbuildup is such that the furnace mustbe shut down for decoking with a consequent loss in production capacity.

The coking problem has been attacked by a process which basicallyinvolves the elimination of the hydrocarbon feed from at least one ofthe tubes and passing therethrough a decoking feed of steam and/or waterwhile maintaining the remaining tubes onstream. Such a process isreported in U.S. 3,365,387 to Cahn et al., which is incorporated hereinby reference. Now, however, by the process of this invention it ispossible to increase substantially the rate of decoking using steam and/or water and thereby return these tubes to onstream service morequickly.

SUMMARY OF THE INVENTION In acordance with this invention, therefore, anim,- proved process of the decoking of steam cracking tubes is providedwhereby the hydrocarbon feed to at least one tube is cut out and adecoking feed comprising steam and/or water and hydrogen is passedthrough said tube or tubes while maintaining the remaining tubesonstream, i.e., in normal service, continuing to crack hydrocarbon feedin the remaining tubes, and maintaining the temperature level of thetube or tubes rbeing decoked essentially the same as the temperaturelevel in the tubes remaining onstream. This invention has the advantageof lending a decoking capability to the steam cracking process whichallows decoking of one or more tubes while maintaining furnacetemperatures and continuing to make product. Additionally, thisinvention contemplates the decoking of only a single tube at a time inthe furnace, thereby not substantially reducing the conversion capacityof the furnace as a whole, or the decoking of any number of tubessimultaneously or successively, or the decoking all of the tubes of thefurnace simultaneously. Nevertheless, because the vapor load ondownstream equipment increases with an increase in the number of tubesbeing simultaneously decoked by this method, it is preferred that only aminor portion of the tubes in a steam cracking furnace :be decoked bythis method at any one time. After the decoking operation has beencompleted, the clean tubes are returned to normal service byreintroducing the hydrocarbon feed, adjusting the steam/water rate, andcutting out the hydrogen.

While not wishing to be bound by any particular theory, it is believedthat the decoking reaction results from an interaction of steam andcoke, according to Equation l:

i.e., the water gas reaction. However, at steam cracking temperaturesthis reaction is quite slow and would not !be expected to beeconomically feasible as a decoking method. However, it is believed thatas the coke forms on the tube surfaces a diffusion process takes placebetween the coke and the metal tube. Thus, some coke goes into solidsolution in the metal tube while trace amounts of metal, e.g., iron andnickel (the latter 1being present in substantial amounts in widely usedstainless steel tubes), diffuse into the coke layer. These trace amountsof metals then tend to catalyze the water gas reaction allowing it toproceed at favorable rates at steam cracking temperatures. Additionally,it is believed that there is an induction period during which the watergas reaction does not proceed rapidly. This induction period is believedto be caused by the presence of sulfur (which is present in some amountin almost every steam cracking feed) which coats the trace elementsthereby masking their catalytic effect. Only when the sulfur is removeddo the trace metals become active catalysts. The process of thisinvention eliminates or substantially decreases this induction perioddue to the presence of hydrogen in the decoking feed, the hydrogenacting as a desulfurization agent:

Hg-l-S (in coke or adsorbed on Ni or Fe)- HZS-l-desulfurizedcokel-l-sulfur free Ni or Fe (2) (Norm-ally, there will be no sulfur inthe decoking feed and the reverse reaction of (2) will not occur.)

The quantities of steam and/or water that are used during the decokingprocess are predetermined to Imeet the following criteria.

(1) Sufficient steam and/or water are introduced to remove the heatnormally going to the process gas, i.e., hydrocarbon and steam mixture,without exceeding the tube metal temperature allowances as determined bystress or oxidation limits for the tube material.

(2) The temperature of the decoking feed, i.e., steam/ water andhydrogen, entering the section of the furnace to be decoked i.e.,cracking section, should be about 700 F. or higher. If water isemployed, it must be vaporized and superheated while steam need only besuperheated.

(3) The mass rate of decoking feed entering the furnace section to bedecoked should preferably be greater than pounds per second per squarefoot of tube internal cross-sectional area when tube outlet pressure isin the order of -25 p.s.i.a. Higher mass rates at constant temperaturereduce the time required for decoking. Higher operating pressures in thefurnace tubes being decoked require higher mass rates of steam for thesame decoking time. (It is noted, however, that in this invention it ispreferable to conduct the decoking operation at pressures as low aspossible, e.g., atmospheric, since lower pressures tend to increase thedecoking rate.)

Operation in this manner insures that the temperature in the tubes beingdecoked is essentially the same as in the tubes remaining onstream,i.e., in normal service.

When decoking is completed, e.g., about 2 to 4 hours for a typical tubeunder optimum conditions, the supply of' steam and/or water and hydrogencan be cut off from the decoked tube and feed may be simultaneouslyreintroduced. The completion of the decoking operation can be monitoredby any one of several methods, such as l) decrease in pressure dropacross the section of the furnace being decoked, (2) decrease in tubemetal temperature, or (3) rate of carbon monoxide formation (cf.Equation l, CO will increase during decoking but fall off sharply whenlittle or no coke is left in the tube).

DRAWING DESCRIPTION This invention may be more fully understood from thefollowing description when read in conjunction with accompanying FIG. lwherein the flow path of the reactants through an apparatus for thermalcracking of hydrocarbons is illustrated diagrammatically.

Referring to the drawing, the cracking furnace 10 comprises an upper,convection or preheat section 11 and a lower, cracking zone 12. Burners13 are provided on the side walls and/or on the bottom of the furnace tosupply heat. The number of burners provided is dependent upon the heatrequired and may vary considerably.

Although not shown in detail in the drawing, the furnace containsseveral conduits or passes in parallel. Each pass may contain a numberof connected tubular members or tubes that provide a flow path throughthe convection section and into the cracking section. ln the drawing,one pass is shown, with the tubes in the convection section 11designated by the numeral 15 and thecracking coils or tubes in thecracking zone 12 designated by the numeral 16. It is to be understoodthat the number of conduits or tubes in the furnace is a function of thesize of the furnace and is dictated solely by design considerations.

Hydrocarbon feed stock is supplied to the steam cracker via supplyconduit 20 and manifold or distributor conduit 21 to the severalparallel cracking conduits or passes. A control valve 22 is provided oneach conduit 23 connecting the feed distributor 21 to each of thecracking conduits or tubes. Steam,`or in the decoking operation, steamand/or water are supplied through inlet line 24and valve 25 to theconduit 23. (In some cases, steam and water are supplied throughseparate lines and not necessarily at the identical point in theconvection section.) Hydrogen is mixed with the steam or water inletfrom line 26 through valve 27.

The reaction products are discharged from the coils or tubes 16 of thecracking furnace via conduits 28 into conduit or header 29 from whichthey are discharged into conduit 30. In order to stop the crackingreaction promptly and thereby prevent or minimize side reactions,quenching agents such as higher boiling hydrocarbons and/or water aresupplied through conduit 31 and control valve 32. The mixture ofquenched reaction products and quenching agent is discharged via conduit30 into fractionating tower 33. Aromatic tar product is withdrawn fromthe bottom of fractionating tower 33 through line 34 and product istaken overhead via line 35. Other intermediate boiling range fractionsmay be withdrawn as product or recycled to a higher plate in thefractionating tower as one or more reflux streams. The quench oil may beWithdrawn from the fractionating tower 33 through line 36 and passedthrough heat exchanger 37 where it is passed in indirect heat exchangerelation to the hydrocarbon feed stock for preheat thereof or to waterfor steam formation while cooling the quench oil to a suitabletemperature for discharge through line 31 and valve 32 into the reactionproduct stream in line 30 as described above.

The onstream decoking procedure requires the closing of one of thehydrocarbon feed valves 22 and the opening of the steam water valve 25.The amount of steam and water passed through the decoking conduit 24 isadjusted so that the steam temperature inside the pass is about 700 F.or higher at the point'of transition from convection'tubes 15 tocracking tubes 16. When sufficient time has elapsed to allow the coke tobe removed from the inside of the tubes, valves 25 and 27 are closed andvalv 22 is opened.

FIG. 2 depicts a graphical portrayal of the increase in decoking ratedue to the presence of hydrogen in a steam/ water decoking feed. Thefigure shows carbon monoxide formation plotted against the mole fractionof water in a decoking feed of water and hydrogen as a function oftemperature. As mentioned, decoking by steam/water involves the watergas reaction and, therefore, carbon monoxide make is directly related todecoking rate. Examination of FIG. 2 shows that an H2O mole fraction ofzero, the rate of decoking is zero. However, for each temperature curveshown the rate of decoking steadily increases as the mole fraction ofH2O increases until a certain point is reached, depending upontemperature, where the effect of hydrogen in the decoking feed begins todrop off and at an H2O mole fraction of above about 0.8 the effect ofhydrogen is negligible and the decoking rate approximates that for wateronly. It is also shown that as temperature increases the optimum steamto hydrogen mole ratio increases. Thus, the steam to hydrogen mole ratiowill vary with temperature. Nevertheless, it can be generally statedthat steam to hydrogen mole` ratios of from 1/1 to 3/1 are preferred andmore preferably 2/1 to 3/1.

These curves were developed by passing a steam-hydrogen decoking feedthrough a coked 310 stainless steel tube of 1" ID by 3 long. The tubewall temperature was maintained at 1600-2000 F. The H2O/H2 ratio wasvaried by mixing CP grades of oxygen and an excess of hydrogen outsidethe tube and combusting the mixture. By varying the rate of oxygen flowthe H2O/H2 ratio could be varied instantaneously and, in turn, allowedthe measurement of the decoking rate, i.e., by carbon monoxide make.

The steam cracking operation is vold and Well known (see, for example,Chemical Week, Nov. 13, 1965, p. 72 et seq), and will only be brieflydescribed hereinbelow. Generally, the petroleum feed fraction is admixedwith steam, i.e., in amounts ranging from about 20-95 mole percentsteam, prior to entry into the steam cracking furnace which may beheated by any suitable means, e.g., gas firing, etc. The furnace itselfnormally contains two sections, a convection section wherein the feed isvaporized, if not already in that form and preheated, and a radiant orcracking section, the feed being passed in admixture with steam throughone or more furnace tubes located within the furnace. The convectionsection is normally employed to increase heating eiciency and thepetroleum-steam mixture is heated therein to intermediate temperatures,i.e., about 100G-1100" F. How.

ever, these temperatures are below that at which the feed cracks sincecracking is undesirable in the convection section. The heated feed thenpasses into the radiant section, i.e., the cracking zone, where thetemperature of the reactants is quickly raised to about 1200-1700 F.,preferably 1500-1700 F., or higher, as tube metal materials permit, andthe feed is cracked. (Generally, raising the temperature of thereactants to the mentioned ranges requires heating the tubes to about1400-2000" F., preferably 1600-2000 F. and higher as tube materialspermit.) Residence times in the radiant section are carefully controlledto minimize polymerization and other undesirable reactions. Thus,residence times in the cracking zone will range from about 0.1-10seconds, preferably 0.1-1 second. Pressures within the tubes may rangefrom about -50 p.s.i.g. but are not critical, and higher pressures e.g.,up to about 100 p.s.i.g. can be tolerated. Upon exiting the crackingzone, the reaction products are immediately quenched to stop furtherreaction and/or minimize loss primary conversion products.

Now, since cracking occurs only in the radiant zone of the furnace, itis only this zone that requires decoking. And, since the flow ofdecoking feed through this Zone will be such as to maintain normalonstream temperatures, the tubes not being decoked can continue to crackfeed with little or no disruption to the entire unit.

The petroleum fractions which may be converted by this process can varywidely and one skilled in the art will readily determine optimumconditions for different petroleum feeds. Generally, however, theprocess is most applicable to hydrocarbon feeds consisting essentiallyof cyclic or acyclic saturated hydrocarbons. Thus, hydrocarbons that maybe utilized herein include such cyclic hydrocarbons as cyclopropane,cyclobutane, cyclopentane, cyclohexane, cyclooctane, cyclododecane,etc., and mixtures thereof. Acyclic hydrocarbon feeds include anyalkane, namely, aliphatic hydrocarbons of the methane series or mixturesof alkanes with cycloalkanes. Preferred feeds are those saturatedhydrocarbons containing from 2 to about 24 carbon atoms, more preferablyalkanes containing 2 to about 12 carbon atoms, e.g., ethane, propane,butane, isobutane, hexanes, heptanes, etc., n-hexadecane, eicosane, andlight naphthas boiling in the range of 90- 430" F., gas oils of 450-800F., or higher boiling points, and kerosenes of 430-550" F. boilingpoints can also be effectively cracked in this process. Because cokingcan be readily controlled by practicing the invention described herein,higher temperatures in the cracking zone (radiant section) can beemployed, the credit for these higher ternperatures being taken asincreased yields or in the cracking of poor quality feed stocks, i.e.,those that would normally give excessive coking.

What is claimed is:

1. In a process for thermally cracking hydrocarbon materials by passingthe same in admixture with steam through a multiplicity of tubesarranged in a cracking furnace wherein the tubes are subjected to heatsufficient to raise the temperature of the reactant so that thereactants are cracked, and wherein coke deposits are formed on theinterior Walls of said tubes, the improvement which comprises taking atleast one of the tubes oifstream by cutting out the ow of hydrocarbonfed and passing a decoking feed containing hydrogen and a componentselected from the group consisting of water, steam, and mixturesthereof, the mole ratio of said component to hydrogen being from about1:1 to about 3:1, through the tubes in suicient amount to maintain thetemperature of the decoking feed in the offstream tubes at essentiallythe same level as in the tubes remaining onstream, thereby effecting theremoval of coke from the interior of the offstream tubes, and thereafterreturning the offstream tubes to onstream operation.

2. The process of claim 1 wherein the hydrocarbon feed is cut out of allof the tubes and the tubes are taken otfstream for decoking andmaintaining the temperature in the tubes at essentially the same levelas that for onstream operation.

3. The process of claim 1 wherein the tubes are heated by radiant heatsufficient to raise the temperature of the reactants within the tubes toabout 12001800 F.

4. The process of claim 1 wherein the hydrocarbon feed is cut out andthe decoking feed is supplied to only one of the tubes at a time toremove coke therefrom without substantially reducing the conversioncapacity of the cracking furnace as a whole.

5. In a process for thermally cracking hydrocarbon materials by passingthe same in admixture with steam through a multiplicity of tubesarranged in a cracking furnace wherein the tubes are subjected toradiant heat sufficient to raise the temperature of the reactants withinthe tubes to about 12001800 F. and wherein coke deposits are formed onthe interior walls of said tubes, the lmprovement which comprises takingat least one of the tubes offstream by cutting out the flow ofhydrocarbon feed and passing a decoking feed containing hydrogen and acomponent selected from the group consisting of steam, water, andmixtures thereof, the mole ratio of said component to hydrogen beingfrom about 2:1 to about 3:1, through the tubes in sufficient amount tomaintain the temperature of the decoking feed in the oifstream tubes atessentially the same level as in the tubes remaining onstream,continuing the supply of decoking feed through the offstream tubes fol aperiod sufficient to effect removal of coke on the interior of theoffstream tubes, and thereafter returning the offstream tubes toonstream operation.

6. The process of claim S wherein a minor portion of tubes are takenoffstream at any one time for decoking.

7. The process of claim 5 wherein the reactants are heated to anintermediate temperature in a convection section of the cracking furnaceand are cracked in a cracking section of the cracking furnace.

8. The process of claim 7 wherein the temperature of the decoking feedin the offstream tubes is at least about 700 F. as it passes from theportion of the tubes in convection section to the portion of the tubesin the cracking section.

9. The process of claim 8 wherein the mass rate of decoking feedentering the portion of the tube to be decoked is greater than 15 poundsper second per square foot of tube internal cross sectional area whenthe tube outlet pressure is of the order of 20a-25 p.s.i.a.

10. The process of claim 9 wherein only one of the tubes is takenoffstream at a time in order to remove coke therefrom withoutsubstantially reducing the conversion capacity of the cracking furnaceas a whole.

7 8 11. The process of claim 10l wherein the decoking feed OTHERREFERENCE 1S sulfur free' R f C t d Comprehensive Treatise on InorganicTheroretical v e "ences e Chemisrry,Me110r,v01. 5, pp. 63 and 64, 1925.

UNITED STATES PATENTS 3,365,387 1/1968 Cahn et aL 208 48 5 DELBERT E.GANTZ, Primary Examiner 2,271,955 2/ 1942 Russell 208-48 J. M. NELSON,Assistant Examiner 2,289,351 7/1942 Dixon et al 208-48 2,547,221 4/1951Layng 252-411 U.S. Cl. X.R.

3,156,734 1/1964 Happel 260-679 208-48

